Excitation energy transfer in a weakly coupled system: Studies with time-resolved fluorescence microscopy and laser induced transient grating techniques

Excitation energy transfer in a new dye pair eosin (donor) to 1,1′,3,3,3′,3′-hexamethyl indotricarbocyanine iodide (HITCI) (acceptor) has been studied in solutions of dimethyl sulfoxide (DMSO) and polymer microparticles by picosecond time-resolved fluorescence microscopy and laser induced transient grating (LITG) techniques. There exits a large shift between the fluorescence maximum of the donor (552 nm) and the absorption maximum of the acceptor (747 nm) with a small spectral overlap resulting in a weak coupling between the two. On increasing the acceptor concentration, the fluorescence intensity and the fluorescence lifetime of the donor decreases in diffusion-controlled fashion. The fluorescence decay curves have been measured by time-correlated single photon counting (TCSPC) technique on ns time scale, while the LITG technique has been used to monitor the short-time decay dynamics. The critical transfer distance calculated from the overlap integral is 34 ± 1 Å. Förster theory of excitation energy transfer has been used to interpret the kinetics at higher acceptor concentrations. When the donor-acceptor system is embedded in single microspheres, while an enhancement in the radiative rate of the donor is observed, but no conclusive evidence of modification of the rate of the excitation energy transfer is found.